In general, inkjet printing machines or printers include at least one printhead unit that ejects drops of liquid ink onto recording media or an imaging member for later transfer to media. Different types of ink can be used in inkjet printers. In one type of inkjet printer, phase change inks are used. Phase change inks remain in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The printhead unit ejects molten ink supplied to the unit onto media or an imaging member. Such printheads can generate temperatures of approximately 110 to 120 degrees Celsius. Once the ejected ink is on media, the ink droplets solidify. The printhead unit ejects ink from a plurality of inkjet nozzles, also known as ejectors.
The media used in both direct and offset (transfix) printers can be in web form. In a web printer, a continuous supply of media, typically provided in a media roller, is entrained onto rollers that are driven by motors. The motors and rollers pull the web from the supply roller through the printer to a take-up roller. The rollers are arranged along a linear media path, and the media web moves through the printer along the media path. As the media web passes through a print zone opposite the printhead or heads of the printer, the printheads eject ink onto the web. Along the feed path, tension bars or other rollers remove slack from the web so the web remains taut without breaking.
Existing web printing systems use a registration control method to control the timing of the ink ejections onto the web as the web passes the printheads. One known registration control method that can be used to operate the printheads is the single reflex method. In the single reflex method, the rotation of a single roller at or near a printhead is monitored by an encoder. The encoder can be a mechanical or electronic device that measures the angular velocity of the roller and generates a signal corresponding to the angular velocity of the roller. The angular velocity signal is processed by a controller executing programmed instructions for implementing the single reflex method to calculate the linear velocity of the web. The controller can adjust the linear web velocity calculation by using tension measurement signals generated by one or more load cells that measure the tension on the web near the roller. The controller implementing the single reflex method is configured with input/output circuitry, memory, programmed instructions, and other electronic components to calculate the linear web velocity and to generate the firing signals for the printheads in the marking stations.
Another existing registration control method that can be used to operate the printheads in a web printing system is the double reflex method. In the double reflex method, each encoder in a pair of encoders monitors one of two different rollers. One roller is positioned on the media path prior to the web reaching the printheads and the other roller is positioned on the media path after the media web passes the printheads. The angular velocity signals generated by the two encoders for the two rollers are processed by a controller executing programmed instructions for implementing the double reflex method to calculate the linear velocity of the web at each roller and then to interpolate the linear velocity of the web at each of the printheads. These additional calculations enable better timing of the firing signals for the printheads in the marking stations and, consequently, improved registration of the images printed by the marking stations in the printing system. Ejection of ink from the inkjet nozzles can be adjusted based on the calculations. A double reflex printing system is disclosed in issued U.S. Pat. No. 7,665,817.
Some continuous feed inkjet printers form printed images on only a first side of the continuous web, a process referred to as a simplex printing operation. Simplex continuous feed inkjet printers have printhead assemblies with printheads that are configured to eject ink across a printing zone on the continuous web that is less than the width of the web. The printing zone is typically centered on the web with appropriate margins on each side of the printing zone. During a simplex printing operation, the continuous web makes only one pass through the printer. Specifically, a rewinder pulls the continuous web through the printer along the web path only once during a simplex printing operation.
Some continuous feed inkjet printers are configured to form printed images on a first and a second side of the continuous web, which is known as a duplex printing operation. In a duplex printing operation, the continuous web makes two passes through the printer, and is referred to as a half-width dual-pass duplex printing operation. In particular, the continuous web is routed from a web supply through the printer to receive ink on the first side. After the continuous web exits the printer, the continuous web is inverted by an inverting system and is then routed again through the printer to receive ink on the second side. One type of duplex continuous feed printer includes an external continuous web inverting system. In another type of duplex continuous feed printer, the web inverting system is incorporated as part of the printer itself. As used herein, the term “inverting”, “inverter”, or “inversion” refers to manipulation of the web to turn the web from a first side to a second side to enable an unprinted side of the web to be presented to a printhead assembly for printing.
In duplex printing on a continuous web, an image on one side is registered with an image on the other side to insure that portions of an image are not lost when the continuous web is cut into sheets or that an image on a first side is not misaligned with an image on a second side. To insure that proper registration is achieved, the speed of the transport rollers is controlled according to a variety of factors including web and environmental humidity, temperature, atmospheric pressure. For instance if the web either stretches, shrinks, or otherwise becomes distorted during transport through the printer or during imaging, poor quality images can occur. Likewise, the amount of ink deposited on the continuous web can affect the material properties of the web and result in misregistration of images as well. Consequently, improvements to a printing system and to printing images by taking into account the types of media, the amount of ink being deposited on the continuous web, and conditions occurring in the printer are desirable.